In general, the greater the number of circuit components that may be placed on a chip, the better the performance. Thus, the density of integrated circuit components in a semiconductor device is closely related to the processing speed of the device. To improve the density, researchers are constantly striving to develop semiconductor chip designs that allow for a decrease in the space between circuit components. Unfortunately, as semiconductor chips become more compact, they also become much more delicate to manufacture and handle.
In most computer systems, these state of the art fine pitch semiconductor chips are mounted to and supported by PCBs. Examples of PCBs include motherboards, graphics, video, and local area network cards. PCBs are also used to support conventional electronic components, such as connectors, capacitors, resistors, and diodes.
Unlike semiconductor chips however, these “old technology” or large pitch electronic components have not been miniaturized, primarily because the components must remain large enough to be installed, removed, or otherwise manipulated by human hands. In addition, connectors must meet agreed upon industrial standards to ensure compatibility between parts manufactured by different parties.
One method for attaching chips and electronic components to the PCB involves the use of printing material or a conductive paste, such as solder paste. In a process commonly referred to as stencil or screen-printing, the paste is deposited or printed onto the PCB through a stencil having a number of apertures in a pre-designed pattern. The stencil is positioned above the PCB so that each aperture exposes areas on the PCB pad intended to receive the paste. After the solder paste is deposited onto the stencil, a squeegee assembly having a squeegee blade is used to push the paste across the surface of the stencil. The blade is typically formed from either polyurethane or metal.
During the printing of the solder paste, the squeegee blade is pressed by the squeegee assembly into contact with and dragged across the surface of the stencil. In some assemblies, the movement of the blade may be controlled not only by direction and speed, but also by rotation and stroke length. The motion of the blade forces the paste through each aperture to print onto the PCB. Because the paste is still wet after it has been deposited onto the PCB, electrical components are easily mounted onto the PCB to complete the circuit design. Finally, the PCB undergoes a reflow process, during which the solder paste is heated and melted to form solder joints.
One of the problems with the previous printed circuit board assembly (PCBA) process is that it is difficult to deposit the right amount of solder paste for all of the different components supported by a PCB. As described above, with the advance of semiconductor technology, not only are chip designs becoming more and more compact, space between the components on a PCB is also rapidly diminishing.
If too much paste is deposited, it may result in a short circuit between nearby components on the PCB. On the other hand, if not enough paste is deposited, the component may not be properly secured to the PCB when it is mounted. Either of these conditions will lead to a greater probability of error, which is conventionally measured by defect parts per million (DPPM).
Because the amount of solder paste depends on the volume of each stencil aperture (length multiplied by width and depth), solder paste volume could traditionally be controlled, simply by varying the width of a stencil aperture. Unfortunately, to achieve satisfactory results for the fine pitch printing required by state of the art components, this method is ineffective. If the width of an aperture is too small relative to its depth, solder paste may stick to aperture walls instead of depositing onto the PCB, resulting in inadequate solder deposit.
Another method of reducing solder paste volume is to decrease the depth of the aperture by reducing the thickness of the stencil. However, in contrast with changing the width of a single aperture, changing the thickness of the stencil would necessarily change the depth of all other apertures on the stencil. If the thickness of a stencil is reduced to accommodate fine pitch components, then it is likely that the stencil will not be able to print enough solder paste to secure large pitch components.
As described previously, there is a tremendous size difference between the largest pitch components and the smallest pitch components that must be mounted on a single PCB. For example, connector pads and capacitor pads have a relatively large surface area from about 10,000 mils2 to about 25,000 mils2. In contrast, the surface area of quad flat packages (QFP) and synchronous dynamic random access memory (SDRAM) pads currently range only from about 300 mils2 to about 600 mils2.
It may be possible to overcome the size difference between the different electrical components by using more than one stencil. However, to avoid complicating the process and incurring extreme costs (e.g. requiring the use of an additional printing machine), it is extremely important for the PCBA process to deposit adequate solder paste volume within a single operation.
In view of the foregoing, it is desirable to have a method and apparatus for increasing solder deposition without increasing the thickness of a stencil. It is also desirable to selectively increase solder deposition while reducing the thickness of a stencil to reduce defects during fabrication of PCBs.